As release agents for pressure sensitive adhesive or release coats for back face of pressure sensitive adhesive tape there have heretofore been used many types of silicone release agents. These silicone-based release agents can be roughly divided into two groups, i.e., condensation or addition type silicone which undergoes ultraviolet curing, EB curing or hot curing to cause condensation reaction or addition reaction on the surface of a substrate and copolymer comprising as a constituent a reactive functional group-containing silicone.
The condensation type silicone can be obtained by the reaction of a dimethyl polysiloxane terminated by silanol functional group at both ends thereof with methyl hydrogen polysiloxane in the presence of a catalyst. The addition type silicone can be obtained by the reaction of a polysiloxane containing a vinyl group at both ends thereof or in its side chains with methyl hydrogen polysiloxane in the presence of a catalyst. These condensation or addition type release agents can be divided into three groups, i.e., solvent type, aqueous type and solvent-free type. These release agents undergo ultraviolet curing, EB curing or hot curing on the surface of a substrate.
Referring to the copolymer comprising as a constituent a reactive functional group-containing silicone, as solvent type materials there have been used compositions comprising as a main component a copolymer of a polyfluoroalkyl vinyl monomer, a silicone vinyl monomer and a copolymerizable vinyl monomer in the presence of a radical apolymerization initiator (JP-A-61-228078 (The term "JP-A" as used herein means an "unexamined published Japanese patent application")). As aqueous type materials comprising similar vinyl copolymers there have been known emulsions comprising as a constituent a silicone-based macromonomer (JP-A-1-318027). Further, block copolymers obtained by the reaction of three components, i.e., an organopolysiloxane containing an alkoxy group, a hydrophilic polyol and a diisocyanate (JP-A-61-28527), aqueous polyurethanes comprising as constituents a polysiloxanediol, an acid diol and other polyols and polyisocyanates (JP-A-5-25239), and modified aqueous polyurethanes containing a perfluoroaliphatic group (JP-A-1-225618).
As mentioned above, silicone release agents or release coats for back face of pressure sensitive adhesive tape exhibit excellent properties such as low releasability and thus have found wide application. However, these release agents leave something to be desired in many respects.
In other words, regardless of whether it is of condensation type, addition type or of the type of copolymer comprising as a constituent a reactive functional group-containing silicone, the solvent in the solvent type silicone release agents raise problems of air pollution, necessity for the improvement of working atmosphere, safety, etc.
Further, the silicone release agents have been found disadvantageous in the art of electronic parts in that silicone migrates to the back face of the tape, deteriorating the adhesivity.
Moreover, the aqueous condensation or addition type release agents which undergo ultraviolet curing, EB curing or hot curing to cause condensation reaction or addition reaction on the surface of a substrate are disadvantageous in that curing cannot proceed completely, resulting in remarkable migration.
The copolymer comprising as a constituent a reactive functional group-containing silicone must be well-balanced between releasability and adhesivity to the surface of a substrate.
Both the silicone polymer-containing emulsion comprising as a main component a copolymer of a polyfluoroalkylvinyl monomer, a silicone-based vinyl monomer and a copolymerizable vinyl monomer in the presence of a radical polymerization initiator and the aqueous polyurethane resin comprising as constituents a polysiloxanediol, an acid diol and other polyols and polyisocyanates are balanced between releasability and adhesivity by properly changing the composition ratio of copolymers. Therefore, in order to control releasability, it is necessary to increase the content of silicone macromonomers. Thus, the control over releasability is limited.
It has been desired to develop a low releasability release agent, a high releasability release agent and a middle releasability release agent. Thus, a technique for freely controlling releasability has been desired. Further, it is necessary to develop an aqueous release agent which undergoes little migration to back face or little change with time.